A conventional method for the production of pigments which are 2,9-dimethoxyquinacridone or its solid solutions involves the conversion of 2,5-di(p-methoxyanilino)terephthalic acid to the desired quinacridone. Color manipulation of the final pigment can then be achieved by modifying selected post-synthetic quinacridone manufacture treatments so as to cause changes in the particle size, particle shape, particle size distribution and/or crystal form of the final pigment. These steps can entail milling procedures (wet, dry, with and without milling aids) and heat treatments (with and without particle growth inhibitors or dispersing aids). It is also known that the use of fully-formed quinacridone derivatives and/or their quinacridone intermediate precursors during the synthetic ring closure procedure to produce the quinacridone itself can be used as a means to alter the color and physical properties of the quinacridone. To those skilled in the art, the term pigment derivative and derivative precursor is well-known. Such derivatives are usually substituted with either a carbonyl, sulfonyl or other connecting functionality which is reacted with an acid, amine, amide, imide, alkyl or alkoxy-containing moiety.
2,5-di(p-methoxyanilino)terephthalic acid is a well known intermediate used in the production of quinacridones. Conventionally, this intermediate is prepared by reacting dimethylsuccinyl succinate and 4-methoxyaniline in a solvent, such as methanol or a higher alcohol, at elevated temperatures in the presence of an acid, and possibly under pressure. The resulting dicondensed material is combined with an oxidizing agent, such as the sodium salt of m-nitrobenzenesulfonic acid, H2O2 or air, and a base, such as NaOH or KOH, and subsequently heated to an elevated temperature, possibly under pressure. The resulting 2,5-di(4-methoxyanilino) terephthalic acid or its metal salt is then diluted with water to obtain a solution. A filtering aid, such as Celite, may be added to the solution and insolubles removed. The resulting solution is acidified with an acid, such as HCl or H2SO4, until the product precipitates.
Polymorphism of the dihydroquinacridone, a quinacridone precursor, and its impact on the resulting quinacridone is well known. Polymorphism exhibited by the quinacridones themselves has also been recognized. For example, P. V. 19's polymorphism is one of the most widely studied and commercialized examples. The polymorphism of other quinacridones, such as for example that of P. R. 122 and P. R. 202, has also been discussed in the literature. However, there is no evidence in the literature that the 2,5-dianilinoterephthalic quinacridone precursors exhibit polymorphism. It has been assumed that these precursors have only one crystal form and that, because the 2,5-dianilinoterephthalic acids supposedly dissolve in the ring closing agent, their physical properties have no impact on the final quinacridone obtained.
It has been discovered that unlike other 2,5-di(anilino)terephthalic acid derivatives, 2,5-di(p-methoxyanilino)terephthalic acid has two distinctly different crystal forms, crystal type I and crystal type II, and that by choosing one of these, the final properties of the quinacridone or solid solution incorporating the 2,9-dimethoxyquinacridone are changed and/or improved.